1. Field of the Invention
The present invention relates generally to a surface acoustic wave (SAW) device, and, more particularly, to a surface acoustic wave device enabling packaging to be effected in a wafer process.
2. Description of the Related Art
Mobile communication equipment such as cellular phones and cordless telephones is rapidly prevailing with recent progress toward miniaturization and lightness of electronic devices. A high frequency circuit of a radio communication circuit included in such mobile communication equipment uses a multiplicity of electronic devices mounted with filter elements.
For the purpose of realizing the miniaturization and lightness in particular, surface acoustic wave (SAW) elements are employed as the filter elements. FIG. 1 is a schematic diagram showing in section a conventional surface acoustic wave device having a surface acoustic wave element, and its wiring connection structure for connecting the surface acoustic wave element to external connection terminals.
A package for the surface acoustic wave device is constructed from a multilayer ceramic package 100 and a metal cap 101. The surface acoustic wave element 104 is adhered by an electroconductive resin 105 to the top of a substrate 103 disposed within the interior of the package, with input and output terminals of the surface acoustic wave element 104 being electrically connected via aluminum wires 106 to a ground terminal. The reverse side of the substrate 103 is formed with an external connection terminal 107.
FIG. 2 shows the structure of another conventional surface acoustic wave device, in which the surface acoustic wave element 104 is connected by connection bumps 108 to the substrate 103 disposed on the bottom of the package, to provide physical fixation and electrical connection wiring.
Thus, in the structure shown in FIGS. 1 and 2, electric wirings (the aluminum wires 106 in FIG. 1 and the connection bumps 108 in FIG. 2) are both formed within the interior of the ceramic package 100.
The cap 101 has a sealing material 109 formed in a region in contact with the ceramic package 100. This provides a hermetic sealing between the ceramic package 100 and the cap 101 so that airtightness is held within the interior of the package.
Thus, to achieve a miniaturization of the surface acoustic wave device, the structure shown in FIGS. 1 and 2 can not neglect the space which is used for the aluminum wire connection and the hermetic sealing structure between the package 100 and the cap 101.
The manufacturing procedure includes making electrode wiring on a piezoelectric substrate wafer by patterning and thereafter cutting and separating the wafer into chip elements to thereby obtain individual surface acoustic wave (SAW) elements 104.
The cut chip elements are mounted on the package 100, which is then fitted with the cap 101 for sealing to obtain a surface acoustic wave device. For this reason, the cost of the cap 101 is a factor greatly affecting the price of product of the surface acoustic wave device. On the contrary, another technique is also known where the package is formed in the state of a wafer (Japan Patent Laid-open Pub. No. 2000-261285).
In the technique described in Japan Patent Laid-open Pub. No.2000-261285, electrodes are formed on a piezoelectric substrate wafer by patterning and a cover forming member is formed from a separate and independent substrate wafer. The cover forming member is then laminated to the piezoelectric substrate wafer having the electrodes formed thereon by patterning, to thereby obtain a surface acoustic wave device having a surface acoustic wave element function.
However, such a technique disclosed in the above patent laid-open publication also imposes a limitation on miniaturization of the surface acoustic wave device and needs a separate provision of the cover forming member, which may be disadvantageous in the number of manufacturing steps. This leads to increase the price of the device.
The present invention was conceived in view of the problems involved in the prior art. It is therefore an object of the present invention to provide a lightweight and chip-sized surface acoustic wave device.
It is another object to provide a surface acoustic wave device capable of being manufactured up to packaging in the state of a piezoelectric substrate wafer by a less number of steps.
In order to achieve the above objects, according to a first aspect of the present invention there is provided a surface acoustic wave device comprising a piezoelectric substrate; a drive electrode unit formed on the piezoelectric substrate, for generating surface acoustic waves; and an electrically conductive electrode protecting unit for covering the drive electrode unit with a hollow therebetween, wherein the electrode protecting unit is formed on the piezoelectric substrate by use of a film forming technique.
In order to achieve the above objects, according to a second aspect of the present invention there is provided a surface acoustic wave device comprising a piezoelectric substrate; an electrode unit formed on the piezoelectric substrate, the electrode unit including a drive electrode unit for generating surface acoustic waves and an external connection electrode unit; an electrically conductive electrode protecting unit for covering the drive electrode unit with a hollow therebetween, the electrode protecting unit being formed on the piezoelectric substrate by use of a film forming technique; an electroconductive column formed on the external connection electrode unit; and an external connection terminal formed at the extremity of the electroconductive column, wherein the piezoelectric substrate is sealed by a resin with the exception of the external connection terminal and the electrode protecting unit.
In order to achieve the above objects, according to a third aspect of the present invention there is provided a method of manufacturing a surface acoustic wave device, comprising the steps of forming a drive electrode having a surface acoustic wave element function on a piezoelectric substrate wafer; resist coating an upper region of the drive electrode; effecting a metal film coating in dome form so as to cover the resist coat; removing the resist lying within the metal dome coated; and providing a resin seal thereon.